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100 MEDICAL JURISPRUDENCE
and MNS. It is transmitted as a mendelian dominant character, and is
present at birth.
The P antigen can be determined with the help of immune rabbit serums
at the same time as MN, but it is quite independent of the latter. Anti-
Pagglutinins have been found to occur naturally in human and animal serums.
All persons can be divided into two main types, viz., P-positive or type P
and P-negative or type pp with the help of anti-P serums. The P antigen
is also inherited as a mendelian dominant character.
None of these antigens can be employed for medico-legal purposes, as
it is difficult to obtain their satisfactory antiserums for the application of
tests. The remaining antigens have not been studied so elaborately as to
be applicable in medico-legal work.
Technique for determining Blood Groups.—The technique for determin-
ing blood groups consists in the use of stock sera of group A and group B
and a 2 per cent suspension in normal saline of the red blood corpuscles
derived from the individual to be grouped. The stock sera should be ob-
tained from a reliable institution and should be fresh and of high titre
A 2 per cent suspension of the blood corpuscles is prepared approxi-
mately by taking a large drop of blood obtained by pricking with a needle
the finger or ear of the individual to be grouped and mixing it with 1 cc.
of normal saline solution in a test tube. A small quantity of 3 per cent
sodium citrate solution should be added to the saline solution before pre-
paring the solution, if it is thought necessary to keep the red blood corpuscles
for more than a few hours before grouping. This suspension may be used
directly or may be centrifuged and the supernatant fluid pipetted off. The
sediment is then suspended in normal saline solution to form a 2 per cent
suspension. After the sera and the red blood corpuscles are ready, the fol-
low" ng method is used for the application of the test: —
A drop of group A serum is placed on one end of a perfectly clean and
dry glass slide, and a drop of group B serum on its other end. A drop of
the red blood cells suspension is added to the serum on each end of the slide
and stirred with a platinum loop. The slide is gently rocked to and fro to
ensure a thorough mixing of the serum with the suspension and is then
allowed to stand for half an hour. After the expiry of this period, irregular
clumps of the red blood corpuscles will be noticed by the naked eye with,
a hand lens or under the low power of a microscope, if haegglutination
is present. These clumps cannot be disturbed on tapping the slide. On the
other hand, in the case of pseudo-agglutination or rouleaux formation, which
is a common phenomenon, the red blood corpuscles are arranged in regular
piles; these can be easily disturbed on tapping the slide.
The group can be determined by observing the following rules : —
If agglutination occurs with group A serum alone, the blood belongs to
group B. If it occurs with only group B serum, the blood belongs to group
A. If agglutination occurs with both the sera, the blood belongs to group
AB. If neither of the sera causes agglutination of the red blood corpuscles,
the blood belongs to group O.
It Is a useful thing to have also a stock of potent serum of group O.
This serum agglutinates the red blood corpuscles of every group except
group O. Group O serum enables one to have a decision when, there is a
poorly marked reaction with either group A serum or with group B serum.
If the red blood corpuscles from such an individual are not agglutinated by
group O serum, the poor reaction with group A serum or with group B
serum is negligible; whereas if agglutination with group O serum occurs,